The present invention relates to a method and apparatus for forming SiC thin films on polymer base materials by plasma (assisted or enhanced) CVD.
In a time in which high-performance globally-environmentally-friendly motor vehicles are desired, the use or adoption of plastic parts in motor vehicles has been promoted with a view toward enhancing weight reduction, thereby the fuel economy of motor vehicles. Notably, thermoplastic resins have attracted attention as an easy-to-recycle material. Accordingly, attempts have been made to positively use thermoplastic resin materials in motor vehicles. Plastic materials, however, have low mechanical strength and surfacial hardness, as compared with those of metallic materials, and are inferior in abrasion resistance compared to metallic materials. Moreover, the discoloration and reduction in hardness of the surfaces of plastic materials are caused by ultraviolet radiation and heat from the sun. Thus, the weather resistance of plastic materials may not be high. Taking the functions and qualities of motor vehicles into consideration, there are limits to which parts can employ the use of plastic materials. Thus, unless the perormance of plastic materials can be enhanced through surface treatment, the promotion and adoption of plastic parts in motor vehicles is not expected.
Heretofore, surface treatment methods of forming a thin film on a plastic material by plasma CVD have been employed. In accordance with plasma CVD, the plastic material can be coated with a high quality film while removing impurities from the thin film as a result of heating a substrate so that the substrate temperature rises to 400 degrees centigrade or more. It was, however, impossible to coat a low heat-resistance plastic material (see S. Wiskuramanayaka, Y. Hatanaka and et al., 1993, Technical Report, Vol. 93, pp. 86-91, Institute of Electronics and Communication Engineers of Japan). Thus, a method has been developed in which plasma treatment is first performed on a polymer base material in the same plasma system by using a non-polymeric gas, such as CO, H2 or O2 gas, so that the adhesive or adhesion strength of the coat is enhanced. Subsequently, a high-durability plasma polymerized coat is formed by performing the plasma polymerization of organic silicides. Although, such a coat is superior in the adhesive strength for adhering to the base material, the coat contains a high proportion of impurities such as carbon and water. Therefore, such a coat has problems in that it lacks high hardness and is inferior in abrasion resistance (see Japanese Patent Laid-Open No. Sho 62-132940/1987 Official Gazette).
On the other hand, when introducing a reaction or source gas into a vacuum reaction chamber, applying a magnetic field (having a flux density) of 875 gausses (G) thereto, and applying microwaves thereto, electrons contained in plasma are accelerated by an electric field caused by the Electron Cyclotron Resonance (ECR) effect, thereby generating a high-density plasma. Furthermore, there has been developed an ECR plasma system utilizing this phenomenon (see Japanese Patent Laid-Open No. Sho 56-155535/1981 Official Gazette).
The aforementioned conventional method, however, uses a large amount of a supply gas. Thus, this conventional method has the following problems. Principly, the operating cost is high. Further, the extent contamination in the system is very high, thus necessitating high costs and large amounts of labor to maintain the system. Moreover, it takes a large amount of time to form a coat. Therefore, a large amount of heat is added to the plastic substrate. Thus, serious damage is caused to the substrate and, consequently, residual strain and cracks occur in the coat.
Thus, a plasma CVD method and apparatus has been developed for forming SiC thin films on polymer base materials by depositing a transparent SiC thin film on the surface of a plastic material through the use of ECR plasma (enhanced) CVD (namely, Electron Cyclotron Resonance Plasma Assisted Chemical Vapor Deposition) techniques, by which the low-temperature deposition of a film of high quality can be achieved, to thereby increase the surfacial hardness of the film without spoiling the designability thereof (see Japanese Patent Application No. Hei 8-52850/1996). In this apparatus, a mesh is placed between a plasma generating chamber and a gas supplying inlet. Further, electrons contained in plasma are trapped by this mesh. The trapped electrons are then made to escape therefrom by way of an electrical ground. Thereby, only radicals (namely, neutrons) are permitted to pass through the mesh. Consequently, the deposition rate is enhanced.
However, even when employing the aforementioned techniques, there is a limit to the hardness of a SiO2 film to be formed as a coat. Thus, there has been a demand for a thin film which has higher hardness. Additionally, SiO2 coats or films transmit ultraviolet light. Therefore, even when such a thin film is formed on a plastic material, there has been concern that the plastic material will be deteriorated by ultraviolet light.
Specifically, SiC film, which have a band gap of about 4.5 to 5 eV, have a smaller band gaps and accompanying ultraviolet cut-off characters, as compared with other SiC films, having band gaps of about 8 eV.
Accordingly, an object of the present invention is to provide a method and apparatus for forming a SiC thin film on a polymer base material by plasma CVD, by which a SiC thin film is provided with sufficient hardness and weather-resistance and resistance to ultraviolet light which can be formed on a plastic base material at a low temperature.
To achieve the foregoing object, in accordance with an aspect of the present invention, there is provided a method for forming SiC thin films on polymer base materials by plasma CVD comprising the steps of: applying a magnetic field to a plasma generating chamber by means of a magnetic coil placed therearound; introducing a microwave into aforesaid plasma generating chamber; introducing an upstream gas into aforesaid plasma generating chamber to thereby generate an ECR plasma; supplying a downstream gas thereto from an inlet; and passing the ECR plasma through a mesh placed between aforesaid inlet and a polymer base material or between aforesaid plasma generating chamber and aforesaid inlet to thereby deposit a SiC thin film on a surface of a polymer base material.
In accordance with another aspect of the present invention, there is provided an apparatus for forming SiC thin films on polymer base materials by plasma CVD, comprising: a plasma generating chamber, to which a magnetic field is applied by a magnetic coil placed therearound, and a microwave is introduced therein, along with an upstream gas introduced therein, for generating ECR plasma; an inlet for supplying a downstream gas therefrom to aforesaid plasma generating chamber; and a mesh placed between aforesaid inlet and a polymer base material or between aforesaid plasma generating chamber and aforesaid inlet.